This expression is supposed to imply a juxtaposition of a crass or offensive action against something that is held as sacrosanct by someone else.

Coined by Ben "Yahtzee" Croshaw

"For four, what the fuck happened to Clive Winston, you pricks?! In his place we've got some multicoloured, giggly J-pop creature, and you can't play classic rock with that...thing. It's like cock-slapping the Mona Lisa."
-Ben "Yahtzee" Croshaw in his Guitar Hero III review